Title: Markers of Exposure to Diesel Exhaust and Cigarette Smoke in Railroad Workers

ABSTRACT

Diesel exhaust is a complex mixture of combustion gases, vapors and particles, and personal exposure can be estimated indirectly only. Quantitative estimates of exposure were developed for thirteen job groups in a large epidemiologic study of mortality among railroad workers. Three possible markers of exhaust exposure were developed. The first index was the concentration of respirable particles because this was simple and inexpensive to measure precisely. Major positive interference, however, was found from environmental tobacco smoke (ETS) and inorganic respirable particles from other emission sources. Composited job group samples were analyzed for particulate nicotine so the ETS component could be subtracted from the respirable particle concentration. This produced a second exposure index, the adjusted respirable particle concentrations. Since there are nondiesel sources of particles in some work areas, a third marker was sought. Diesel exhaust particles have a relatively high content of dichloromethane extractable matter, but inorganic particles have a low extractable content. Therefore, the air concentration of extractable mass was used as a third marker of diesel exposures. The extractable matter also was corrected for the contribution of ETS. The advantages and limitations of these three markers are of interest. In general, considerable caution should be used in the development and application of markers; their use requires detailed knowledge of the nature and sources of exposure in a given setting.

EXPERIMENTAL MATERIALS AND METHODS

Over 550 samples of respirable particles were collected from the breathing zones of workers in 13 job groups at 4 small railroads between 1981 and 1983. In addition, high-volume respirable particle samplers at fixed locations collected 23 samples of diesel exhaust generated by railroad locomotives
operating intermittently in the “running repair” shops. This diesel exhaust had aged for several minutes to several hours and was identical to the exhaust breathed by the workers. Details of the collection of personal samples from the rail-road workers are presented elsewhere.

Personal samples of respirable particulate matter were collected on preweighed and preextracted, 37-mm diameter Teflon®-coated glass-fiber filters (Emfab TX40 H120 WW, Pallflex Products, Inc., Putnam, Conn.) at 1.7 Lpm with a 10-mm cyclone to remove nonrespirable particles (3.5 pm aerodynamic diameter, 50% cut point, geometric standard deviation 1.5). After sampling, filters were stored in Teflon-lined tin cannisters on dry ice or i na freezer until 24 hr prior to reweighing, when they were allowed to equilibrate with room temperature and humidity, The samples and extracts
were stored at -20°C while awaiting processing or analysis.

High-volume samples of diesel exhaust were collected on 20.4 * 25.4-cm Teflon-coated glass-fiber filters (EMFAB TX 40 H120 WW, Patlflex Products, Inc.). Total and respirable
particles were collected with high-volume filter samplers (General Metal Works Model 2000, General Metal Works, Cleveland, Ohio); the respirable particle sampler was equipped with an Aerotec Model 11 (UOP, Darien, Conn.), staintess-steel cyclone preseparator operated at 430 Lpm. The filters
were prepared and stored according to the same protocol as the personal filter samples.

Eight samples of ambient cigarette smoke in an exposure chamber were collected with the same sampling system as the personal samples from the railroad workers. The cigarette smoke was generated in a 34 m® environmental chamber, with aluminum walls and ducts, operated at the Pierce Foundation Laboratory (New Haven, Conn.). Careful control of temperature (23°C), humidity (50% RH), and airflow
ensured the air was conditioned and well mixed. In the room, 4 people sat and smoked at a constant rate; 1 cigarette was smoked every 7.5 min, so a steady-state smoke concentration was established during the sampling time. Samplers (N = 8) were placed at various locations around the chamber,
at least 1 m from the nearest smoker. Samples were collected over a 4-hr period. Further details of the chamber are presented (see Reference 10).

Extraction

Figure 1 outlines the overall analysis scheme followed for freight conductors; each job group was handled in a similar manner. Filters were weighed individually and then composited by job group within each railroad for further analysis. These 47 composite samples were extracted successively
with 3 aliquots of 15 mL dichloromethane and ultrasonicated for 15 min each time. The aliquots were combined, filtered through a 0.45-um pore size Millex HV filter (Millipore Corp., Bedford, Mass.), and concentrated by evaporation under a stream of nitrogen. To ensure sufficient material for precise analysis, 5 filter samples of ETS from the environmental chamber experiment were composited, extracted, and analyzed according to the same protocol within 3 weeks of collection. Four months after sample collection, 3 others were individually extracted and analyzed.

One-half of the extract from each composite of personal samples was reserved for nicotine analysis, except extracts from Railroad IV which had been used previously for PAH analysis. The other half of the extract was placed in a pre~ weighed, Teflon weighboat and allowed to evaporate to constant weight in a box flushed with nitrogen to determine the percentages of extractable material in the particulate matter of the composited samples. For each job group, the personal composites from all railroads were combined in the same weighboat (see Figure 1) to have sufficient mass to
determine the percent of the RSP which was extractable in each of 12 job groups. The weighboat then was reweighed on a Cahn 21 electrobalance (Cahn, Cerritos, Calif.). Several composites of blank filters also were extracted, filtered and weighed. The average mass for these blanks was subtracted
from the mass extracted from each of the composite samples. In addition, an aliquot of the composite of 5 cigarette smoke samples was weighed to determine the extractable percent of ETS. The precision of this procedure was determined by making replicate measurements of percent extractable from
high-volume filters which were split and analyzed separately; they agreed with a mean difference of 2.2% (+ 1.9%). The relative precision for the personal samples could not be measured, but would be somewhat poorer because they contained less particulate matter than the high volume samples.

Nicotine Analysis

The reserved aliquots of the sample extracts were concentrated further to about 50 uL and spiked with naphthalene and chrysene as internal standards. Analysis was performed on a 30-m, DB-S fused silica capillary column in a Shimadzu-6AM gas chromatograph equipped with a flame ionization detector. The initial temperature of 60°C was increased at 2°C/ min to 243°C, and then held for 10 min. Chromatograms were recorded and integrated by a Hewlett Packard 3390 integrator (Hewlett Packard, Palo Alto, Calif.). Nicotine standards were prepared fresh daily from pure nicotine
(Eastman, Rochester, N.Y.) dissolved in dichloromethane. Standards at 5, 10 and 20 wg/mL plus a blank were run by autosampling overnight so that each set of 2 or 3 samples was preceded and followed by standards.

RESULTS

Over 500 personal samples of respirable particles were collected at 4 railroads. The air concentrations of respirable particulate mass (RSP) in all the railroad jobs were low by normal industrial standards, generally less than 300 ug/ m3.  The mass of respirable particles collected on these personal samples rarely exceeded 500 ug and generally was less than 200 ug. Thus, there was potential for cigarette smoke to be a significant part of the particulate mass.

Environmental Tobacco Smoke

Eight samples of environmental tobacco smoke (ETS) were collected from the environmental chamber. Each filter collected an average of 318 +- 35 ug respirable particles and 3.44+ 0.80 ug nicotine. No  difference was found between samples extracted and analyzed immediately and those stored for 4
months before analysis. Overall, each microgram of nicotine was associated with 92.4 ug of cigarette smoke particles.

The composites of the personal samples within jobs at each railroad yielded between 0.1 and 15 ug nicotine. The ETS particle concentration was calculated according to Equation 1:

Average ETS concentrations for each job category within each railroad are presented in Table I. As might be expected, a wide variation was found both from job to job and from railroad to railroad within a given job because these values reflect the influence of different individual smoking patterns and variable environmental conditions. This variability most clearly is seen in the mean exposures of the clerks across the 3 railroads: 132 ug/m3 at Railroad I; 86 ug/m3 at Railroad II; and only 45 ug/m3 at Railroad III. These average levels were much greater than those for most other job
groups, where exposures tended to be between 15 and 50 ug/m3. ETS concentrations also were high (over 90 ug/m3) at Railroad III for the freight conductors and shop workers in the winter. These mean values were not calculated from a range of individual exposures but were the values measured
for sets of composited samples, and so some individual exposures could be much higher or lower.

Adjusted Respirabie Particulate Concentrations

The ETS particle concentration for a job group was subtracted from the RSP concentration to obtain an adjusted respirable particulate (ARP) concentration for that group. In Table II the concentration of RSP found for each of the job groups, combined across three railroads, is compared
with the concentration of ARP. Removal of the ETS contribution made a marked difference for some jobs, such as clerks, but little difference for others, such as signal maintainers and repair shop workers.

Dichioromethane Extractable Mass

The components of sand and gravel dust are not extracted with dichloromethane, but much of diesel exhaust is, including the mutagens and carcinogens. For most jobs about 40% of the mass was extractable (Table III). Fixed location, high-volume samples of RSP that were collected in the
repair sheds averaged 44% extractable; these samples contained predominantly diesel exhaust aged for several minutes to several hours. The personal samples of respirable particles from the breathing zones of machinists and electricians—who work in the repair shop—also contained about 40% extractable material, as did those samples from the breathing zones of engineers and firers. By contrast,
samples from the brakers and hostlers contained less extractable material; only about 20% of the RSP was extractable. Brakers walk on the gravel roadbed beside the trains and locomotives and so are exposed to the sand, dust and gravel mechanically aerosolized by the passage of the train or the
workers’ activities. Hostlers service the locomotives and load them with sand. On the other hand, extractable mass was found to be greater than 40% in the samples from clerks and conductors. As seen in Table I, these samples also contained the highest amount of ETS, and the ETS particles contain a large fraction (61%) of extractable material (Table II). Therefore, the ETS in these samples contributed to the high extractability of the samples from the clerk and conductor job groups.

Adjusted Extractable Matter Concentration (AEM)

The amount of extractable matter in the suspended particles provides a third index of exposure to diesel exhaust. Since cigarette smoke also contains substantial amounts of extractable material, however, particle extracts also must be adjusted for the presence of extractable cigarette smoke
components. The average adjusted extractable material (AEM) concentration was calculated for each job group by using the average fraction extractable and the cigarette smoke content measured on composite samples according to Equation 2:


In Table 1I the values of AEM concentration calculated for each job group are compared with the other indexes of exposure to diesel exhaust, RSP and ARP. A 6-fold range in AEM levels was found across job groups-—from clerks witha bout 7 ug/ m3 exposure to repair shop workers with exposures around 45 ug/m3.

